Conventional electro-acoustic transducers can be roughly classified into speaker systems comprising a combination of functional units in charge of reproduction in specific frequency ranges such as low, middle and high frequency ranges; general speakers of which a single unit can perform reproduction in entire frequency ranges; micro-speakers for performing reproduction in possibly entire frequency ranges within portable electronic equipment; and receivers and earphones for performing reproduction only in a voice frequency range.
Although there are no prescribed specific criteria for classifying the transducers into general speakers and micro-speakers, it is common to classify them based on external sizes and heights of units themselves thereof. In addition to such classification based on external dimensions, more specific features for clearer classification such as the number of suspensions as damping structures of vibration systems and connection structures from electrode terminals to voice coils in speakers are used as classification criteria. Here, the number of additional suspensions for damping a vibration plate is increased in case of general speakers for performing reproduction in entire frequency ranges in order to transmit sounds in a general free sound field, or a high-functional speaker for large input and extended reproduction of bass sounds. As for connections for supplying electrical signals from terminals to voice coils, soft and resilient copper foils capable of accepting a vibration force from a vibration system, and silk wires constructed by twisting cotton or synthesized threads are used additionally. Even in this case, a great number of copper foils and wires are used for high-functional speakers requiring large input and extension of bass sounds. However, as for micro-speakers that are mainly used in near sound fields and employed in products with very limited spaces for accommodating speakers, including cellular phones, notebook computers and camcorders, it is impossible to use the parts employed in such general speakers, i.e. additional copper foils and silk wires, due to very narrow spaces that will be occupied by parts.
FIGS. 1a and 1b are views showing the structure of a general micro-speaker.
Such a conventional micro-speaker comprises a magnetic circuit for generating magnetic flux, a vibration system that vibrates due to repulsive force against the magnetic flux acting on the magnetic circuit, and a main body. Here, the magnetic circuit comprises a permanent magnet 8, a yoke 9 with the permanent magnet 8 contained therein, and an upper plate 7 attached to an upper surface of the permanent magnet 8. Further, the vibration system comprises a voice coil 5 fitted into a gap between the permanent magnet 8 and the inner diameter of the yoke 9 and wound up to generate the magnetic flux when an electric current flows thereinto, a vibration plate 2 integrally bonded to the voice coil 5, and a first suspension 3 (also referred to as so-called “edge”) extending integrally from an outer periphery of the vibration plate 2. The main body comprises a frame 6 to which the magnetic circuit and the vibration system are fixed, an electrode terminal portion 10 fixed to the bottom of the frame 6 for supplying electric power to the voice coil 5 of the vibration system, and a protector 1 fixed to the top of the frame. The yoke 9 is formed with vents 12 for transmitting sounds.
In such a conventional micro-speaker, the vibration of the vibration plate is damped only with the single suspension 3 integral with the vibration plate. Further, the conventional micro-speaker employs a structure in which the connection of the electrode terminal portion 10 to the voice coil 5 is made by using a residual wire 11, which remains after winding up the voice and is then shaped into a semicircle. For this reason, compared with conventional large infinity speakers, the conventional micro-speaker is subjected to the occurrence of a large amplitude of eccentric vibration accompanied with abnormal sounds throughout the vibration plate since the damping capability of the vibration system is weakened as electrical energy input into the vibration system is increased. Further, since the electric power is supplied through two strands of the residual wire of the voice coil mainly composed of copper, considerable progressive damage naturally occurs as the vibration of the vibration system becomes excessive. This means that as the input energy increases, a distortion phenomenon of reproduced sounds accordingly becomes severer and there is a high possibility of the occurrence of progressive disconnection of the wire.
Moreover, since most of recently sold cellular phones employ 40 poly tones including musical components, the number of electronic equipment requiring wideband reproduction and reproduction of a great deal of sounds is increasing day by day. Therefore, there is a need for a micro-speaker with input power of 1 Watt. Referring to micro-speakers of 17 mm in size that have been mainly used as micro-speakers for cellular phones in die relevant industry, however, allowable input power is merely about 0.5 to 0.6 Watts in case of a height of 4 to 5 mm and most of them reproduce bass sounds of frequencies over 750 Hz.
The fact that a limit frequency of reproduction of bass sounds in the micro-speakers is 750 Hz or higher means that there are no lingering effects of bass and medium-pitched sounds. Therefore, the conventional micro-speakers reproduce only sharp and noisy sounds excluding softness and vividness from the overall reproduced sound quality. Here, if the limit frequency of reproduction of bass sounds is extended to a lower frequency range in a relevant speaker unit in order to extend a frequency range of reproduction of sounds to a low frequency range, the amplitude of vibration in an extended resonance range of bass sounds becomes larger than that before the extension is made, even though the same energy is supplied thereto. For this reason, substantial increases in the distortion and the possibility of the occurrence of disconnection of the wire become much higher.
If the limit frequency of reproduction of bass sounds in a single unit of such a conventional micro-speaker of 17 mm in size which employs a processing method using a single suspension and a voice coil is required to be 400 Hz in consideration of some degree of lingering sounds and a balance of reproduction ranges, the aforementioned problems such as disconnection of the wire and abnormal sounds occur considerably. Accordingly, it is indispensable to greatly reduce the allowable input power to 0.2 to 0.3 Watts in case of the conventional micro-speakers. As a result, reproduced sounds become very small sounds. Consequently, to make micro-speakers having large input power centered on reproduction of a great deal of sounds, the limit frequency of reproduction of bass sounds is necessarily shifted to a range of high-pitched sounds with a small amplitude of vibration as near as possible. In a case of making the micro-speakers be centered on reproduction of high-pitched sound, the allowable input power should be lowered. Thus, the conventional micro-speakers have already exhibited vulnerability and limitation due to an inverse correlation between the reproduced sound quality and the allowable input power.
Therefore, it cannot be expected to remarkably improve the function of such conventional micro-speakers. Consequently, to implement a highly reliable micro-speaker that enables wideband reproduction through high level acoustic output resulting from large electrical input and through extension to a range of bass sounds, it is inevitable to provide an additional innovative damping system having a stable damping function even though vibration is increased, and a vibration adaptive, electrical signal-transmitting structure in which disconnection of a wire does not occur.
However, in view of the aforementioned external features of the very small and ultra slim structure of the existing micro-speakers, it is impossible for the micro-speakers to employ an additional suspension or silk wires as vibration adaptive, electrical signal-transmitting bodies, which are made of cotton or synthesized fiber and then woven and thermally molded and are included in general infinity speakers. Accordingly, there is a need for a micro-speaker that includes a structure having the function of the suspension and the silk wires and simultaneously having two functions of vibration damping and wire disconnection prevention while maintaining the concept of the existing micro-speakers in external dimensions.